Thanks Vanessa!
The phone came with Android 4.0.1. but I did a firmware update (through KIES) and it went to 4.1.1. My original T-Mobile T999 had been 4.3.
Perhaps naively, I bought the phone on eBay because it was listed as "New Samsung Galaxy S3 i747 UNLOCKED T-MOBILE NET-10 16GB 8MP WHITE S III" . If it had been correctly listed as an "unlocked at&t", I would not have bought it.
Vendor (cellcycler) has said they will take it back and fix the listing name. Their tech support has been very helpful in working with me to try get it working. But the bottom line is that I expected it to be plug and play since it was advertised as a T-Mobile. I did not do my homework so I did not realize that the i747 is not a T-Mobile model.
My old T999 still works fine (if there is such a thing with minor glass cracking in two corners). So I may just hang onto it until I can afford a more current phone. It would be nicer to have LTE, but the 4G speed on my T999 is not bad.
Thanks again!